hard life choice

Ok, i know this is not likely the right way or anything, but i dont know what to do, ive been in a relationship for 8 yrs now and at first it was good, and than every yr just seems to get worse, its not like shes doing anything wrong either, were just to different i guess i dont know, hardly any romantical interactions, so (this is where i know im a terrible person) i found it with someone else, at first it was just for the close comforts but of course now we have feelings, and time has passed, i ended that and than 8 months later started it back up with her, i know i have to end one, but this is what i dont know, i know ill be happier with the other girl, since ive been unhappy for yrs, but i dont know how to break that to my 8yr relationship without breaking her heart in the meantime, i really care for her and dont want to hurt her, which i know the cheating was obviosly stupid, she does not know about it, i cant talk to anyone i know about it because were all mutual friends, i need outside advice of how to do this gently as to not hurt her,